Teaching electric circuits with multiple batteries: A qualitative approach

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  • Teaching electric circuits with multiple batteries: A qualitative approach

    David P. Smith* and Paul van Kampen

    Centre for the Advancement of Science and Mathematics Teaching and Learning, and School of Physical Sciences,Dublin City University, Glasnevin, Dublin 9, Ireland

    (Received 17 August 2011; published 15 November 2011)

    We have investigated preservice science teachers qualitative understanding of circuits consisting of

    multiple batteries in single and multiple loops using a pretest and post-test method and classroom

    observations. We found that most students were unable to explain the effects of adding batteries in single

    and multiple loops, as they tended to use reasoning based on current and resistance where reasoning based

    on voltage is a necessity. We also found that problems such as thinking of the battery as a source of

    constant current resurfaced in this new context, and that answers given were inconsistent with current

    conservation. We describe the curriculum we developed that enables students to model circuits with

    multiple batteries qualitatively. Post-test results show that the majority of students were able to apply their

    newly developed model to make accurate predictions for complex circuits.

    DOI: 10.1103/PhysRevSTPER.7.020115 PACS numbers: 01.40.Fk, 01.40.jc, 01.50.Qb

    I. INTRODUCTION

    In the past three decades, there has been a great deal ofresearch on the teaching and learning of electric circuits,with many different approaches taken [15]. At universitylevel, research has mostly concentrated on the qualitativereasoning of preservice science teachers and students inintroductory physics courses. The findings of this researchhave highlighted a number of student misconceptions, in-cluding the difficulty of distinguishing between voltage andcurrent. Indeed, many students think that voltage is a prop-erty of current [6]. Sometimes, common predictions that areincorrect for voltage hold true for current: for example,von Rhoneck [7] reported that students predict zero voltageacross an open switch and a nonzero voltage across a closedswitch. Several studies have shown that, even at quite anadvanced level, students treat current in purely resistivecircuits as the primary concept, while voltage is regardedas a consequence of current and not as its cause [8].

    Out of this body of research two main instructionalstrategies have evolved. Both strategies separate the con-cepts of voltage, current, and resistance. One deals withvoltage as the primary concept, the other, current.Examples of the former approach include Cohen et al.[1], Psillos et al. [3], and Rosenthal and Henderson [5].Typically, energy transport and (electron) current aredistinguished quite early on. Approaches followed by

    McDermott et al. [9] and Tiberghien [10] focus on currentfirst and delay the introduction of voltage.Other strategies include the comparison of voltage and

    current to pressure and water flow. The water analogy oftengoes little deeper than the somewhat ad hoc assertions thatvoltage is just like pressure and in parallel circuits,current splits into two, just like water does at a fork in ariver. However, Schwedes and Dudeck [11] developedcurriculum where students develop a complete analogicalmodel for water flow in a closed loop system, using adouble water column of constant height difference as avisual analogy for a constant voltage battery, clampedtubes as variable resistors, and water flow meters as lightbulbs. Electric circuits are then introduced in analogy withthe water loop model.Although all approaches deal with the concept of volt-

    age in some detail, they do not put much emphasis on whathappens to circuits when the number of batteries is in-creased. Yet, there is some important elementary physics tobe gleaned from considering such circuits, as they mayserve to introduce more advanced concepts such as poten-tial (as distinct from potential difference) in a straightfor-ward manner [12]. Curriculum on multiple batteries alsoaffords students another opportunity to differentiatebetween current and voltage. In this paper, we discusscurriculum on the addition of batteries in various waysthat achieves both of these goals and leads naturally to aqualitative treatment of Kirchhoffs loop rule in multiple-loop circuits.While most introductory university physics textbooks

    provide a quantitative treatment of Kirchhoffs rules, thedevelopment of a coherent conceptual framework thatallows students to consider multiple-loop multiple-batterycircuits without recourse to solving equations is typicallynot a goal of instruction. Often, circuits are given with anumber of linear resistors and batteries configured in a

    *Present address: Physics Education Group, University ofWashington, Department of Physics, Box 351560, Seattle, WA98195, USA.

    Paul.van.Kampen@dcu.ie

    Published by the American Physical Society under the terms ofthe Creative Commons Attribution 3.0 License. Further distri-bution of this work must maintain attribution to the author(s) andthe published articles title, journal citation, and DOI.

    PHYSICAL REVIEW SPECIAL TOPICS - PHYSICS EDUCATION RESEARCH 7, 020115 (2011)

    1554-9178=11=7(2)=020115(10) 020115-1 Published by the American Physical Society

    http://dx.doi.org/10.1103/PhysRevSTPER.7.020115http://creativecommons.org/licenses/by/3.0/

  • small number of loops, like the circuit of Fig. 1(a).Students are encouraged to label the currents through allloops, assume a direction, and apply Kirchhoffs loop andjunction rules. Many of the circuits that appear resemblethose selected by Woodman [13] and allow for a straight-forward stepwise solution of the many equations withmany unknowns that emerge. Nevertheless, the emphasisis on developing techniques to find a numerical solutionrather than on developing a conceptual framework basedon qualitative reasoning. For example, textbooks can seemto encourage students to randomly assume directions forcurrent at different junctions, and if the sign of a currentcomes out negative, this is typically shrugged off as incon-sequentialwe just assumed the wrong direction. (Thisis of course different from situations where studentsthrough qualitative reasoning predict the direction of thecurrents, in which case a negative sign provides a valuablenumerical check on previous reasoning.)

    When dealing with linear resistors only, the absence ofbuilding a conceptual framework could be seen as noworsethan an opportunity missed. However, circuits containingnonlinear resistors such as bulbs cannot be treated in thisway. For example, to analyze the elementary circuits ofFigs. 1(b) and 1(c) a qualitative approach becomes a ne-cessity. To illustrate the point, the currents in the circuit ofFig. 1(a) are readily found to be i1 V0=R1, i2 V0=R2,and i3 V01=R1 1=R2. Given constant values for V0,R1, and R2, straightforward substitution will yield numeri-cal values for the currents. When R1 R1i1 and R2 R2i2, as is the case for bulbs, values for the currents andresistances can be found through an iterative procedure, butonly if the i; V characteristic of the bulb is known.Therefore, if the non-Ohmic behavior (i.e., current-dependent resistance) of bulbs is to be taken into account,a qualitative approach becomes a practical necessity.

    In this paper, we describe a research-validated approachto the teaching and learning of circuits containing multiplebatteries and bulbs in multiple loops. In Sec. II, we de-scribe the prerequisite knowledge and understanding ofsimpler electric circuits and the prior exposure of ourstudents. In Sec. III, we describe student difficulties withmultiple batteries in single loops, the curriculum developedto address these, and post-test results. Section IV detailspretest results, curriculum, and post-test results for mul-tiple batteries in multiple loops.

    II. PREREQUISITES

    The curriculum we developed was used to supplementparts AC of the Electric Circuits curriculum in Ref. [9]. Ithas been used in courses for preservice and in-serviceteachers at Dublin City University (DCU), the Universityof Washington, and the University of Maine. At DCU thegroups were sufficiently large that meaningful pre- andpost-test data could be obtained. Typically, the pre- andpost-test questions were answered by 3050 students,which is not a large enough cohort to allow for a robustquantitative analysis. Nevertheless, we think it is mean-ingful to indicate the prevalence of certain answers inpercentages rounded to the nearest 5%; however, onemust bear in mind that the error bars are larger thansuggested by this practice.The curriculum on multiple batteries in multiple loops

    should be usable as a stand-alone provided the studentsmodel for electric circuits incorporates the followingelements:(1) There is a current in complete circuits. This current

    is not used up.(2) The magnitude of the current through a battery

    depends on the resistance of the circuit. When theresistance of the circuit increases, the currentthrough the battery decreases, and when the resist-ance of the circuit decreases, the current through thebattery increases.

    (3) Kirchhoffs junction rule, including the rule thatin a single-loop circuit the current is the samethroughout.

    (4) The voltage across the terminals of a battery isconstant. (In practice, even when low-resistancebulbs are used with 1.5 V D batteries, the deviationfrom ideal behavior is negligibly small.)

    (5) Kirchhoffs loop rule applied to single-batterycircuits.

    (6) The brightness of a bulb increases when the currentthrough it increases. The brightness of a bulb alsoincreases when the voltage across it increases. Thus,bulb brightness links current and voltage qualita-tively. Ohms law does not apply to bulbs.

    Another useful skill, further developed in these labs, isthe ability to transfer between circuit diagrams and physi-cal circuits in the laboratory.As we will demonstrate in Secs. III and IV, even students

    who have a good grasp of these concepts (as evidenced bypost-testing) typically do not correctly generalize these tocircuits with multiple batteries.

    III. MULTIPLE BATTERIES IN A SINGLE LOOP

    In this section, we discuss common student difficultieswe identified in relation to circuits containing more thanone battery. The batteries are connected either in series orin parallel with each other (but not in parallel with other

    (a) (b) (c)

    V0

    V0 V

    0

    R1

    R2

    i2

    i3

    i1

    FIG. 1. Circuits with multiple batteries in multiple loops.Standard methods can be used to solve problems with linearresistors like (a), but fail to deal with nonlinear resistors as incircuits (b) and (c).

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  • circuit elements). For brevity, we say that these circuitscontain multiple batteries in a single loop. We also discussthe curriculum we developed to address these difficultiesand analyze post-test results.

    A. Multiple batteries in seriesPretest analysis

    Before we administered the curriculum, we gave stu-dents a pretest question pertaining to the circuits of Fig. 2.The question examines student understanding of how theaddition of batteries in series in different orientationsaffects bulb brightness. It also implicitly addresses under-standing of series circuits in a new context.

    Only 3 students out of 39, i.e., about 10%, ranked thebulbs correctly by brightness (A B C> D E). Themost prevalent incorrect answers are listed in Table I.Some 55% of students state that bulb A will not light.Without exception, they reasoned that the battery in oppo-site orientation blocks the current or must have a reallyhigh resistance. In many cases this reasoning appears tostem from experiences with putting batteries the wrongway into household appliances. However, this reasoningwas not applied in a way that is internally consistent, orconsistent with the model described in Sec. II, as smallerfractions of students predicted that bulb D (50%) and bulbE (35%) will not light.

    It is also worth noting that 10% of students rank bulb Cbrighter than bulb B, or bulb E brighter than bulb D.Analysis of students answers reveals that it is not thecase that the old misconception of current being used upresurfaces; instead, these students reason that the batteriessupply different amounts of current to different bulbs orthat there can be no current flow between two negativeterminals. In doing so, they appear to revert to ideas of

    batteries supplying current to empty wires and the currentstopping and starting at the battery terminals, and abandonthe notion that the current is the same at every point in asingle loop.

    B. Multiple batteries in parallelPretest analysis

    To test students initial ideas on batteries connected inparallel with each other, they were asked to rank thebrightness of the bulbs and the currents through the bat-teries in circuits (a)(c) of Fig. 3. The question also allowsus to probe to what extent the prior instruction described inSec. II had been successful.Table II summarizes the answers given. Of the six

    students (20%, N 27) who ranked the bulbs correctly(A B> C D), three did not provide a reason for whythey ranked bulbs A and B brightest, and two others merelystated that the circuits of Figs. 3(a) and 3(b) contained justone bulb. Only one student explicitly gave correct reason-ing, based on voltage.It is more instructive to consider partial rankings. We

    found that 25 students (95%) correctly ranked bulbs C andD as being equally bright with correct reasoning. Likewise,95% ranked bulb B brighter than C, with correct reasoning.With just one exception, all reasoning was based on currentand resistance, and not on voltage. We regard these resultsas evidence that the prior instruction described in Sec. IIhas been successful.Seventeen students (65%) ranked bulb B brighter than

    bulb A. One of these students based her reasoning onvoltage; she thought the voltage across bulb B would betwice that across bulb A. The other 16 based their reasoningon the idea that each battery supplies a current to the circuit;

    TABLE I. Common incorrect answers to multiple batteries inseries pretest question of Fig. 2.

    Incorrect answer Pretest (N 39)Bulb A will not light 55% (21)

    Bulb D will not light 50% (19)

    Bulb E will not light 35% (14)

    Bulb C is brighter than bulb B 10% (3)

    Bulb E is brighter than bulb D 10% (3)

    1 2 3 4 5A B

    C

    D

    (a) (b) (c)

    FIG. 3. Circuit diagrams used in multiple batteries in parallelpretest. Students are asked to rank the bulbs by brightness andthe batteries by current, and explain their ranking.

    TABLE II. Common rankings for multiple batteries in parallelpretest question of Fig. 3. In all tables, bold font indicates correctanswers.

    Battery ranking (N 27) Bulb ranking (N 27)1> 2 3> 4 5 10% (3) A B> C D 20% (6)2 3> 4 5> 1 15% (4) B> A> C D 25% (7)Partial rankings: B> A C D 20% (5)2 3, 4 5 50% (13) Other 35% (9)2< 3, 4< 5 20% (6)2> 3, 4> 5 10% (3)

    A B CD

    E

    FIG. 2. Circuit diagrams used in multiple batteries in seriespretest. Students are asked to rank the bulbs by brightness, andexplain the reasoning behind their ranking.

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  • it appears that they implicitly assumed that this current isindependent of the circuit. In classroom discussions someof these students introduce ideas akin to superposition. Thisassumption leads them to believe that bulb B is brighterthan bulb A since it is supplied current from two batteriesrather than one. This generalization satisfies the junctionrule but is inconsistent with reasoning based on voltage(either via the loop rule or observed bulb brightness). Thethree students who thought bulb A would be brighter thanbulb B did not explain their reasoning, and one studentsanswer did not rank bulb A relative to bulb B. Our findingshere are similar to those documented by Licht [14].

    The 27 students gave 18 different rankings for thecurrent through the batteries. The six students whocorrectly ranked the brightness of the bulbs in Fig. 3(A B> C D) gave six different rankings for the cur-rent through the batteries; none were consistent with theirbulb rankings. When these students were trying to makesense of the unfamiliar circuits, they did not treat thejunction rule as inviolable.

    Just under half of the students (13 out of 27) correctlystated that the currents through batteries 2 and 3 on the onehand, and 4 and 5 on the other hand, would be equal. Theyappeared to use symmetry reasoning. Six out of 27, or20%, thought that the currents through the batteries drawnclosest to the bulbs would be greater. Their reasoningtended not to be sufficiently detailed to draw any firmconclusions, but it could be the reappearance of an oldnotion some of them may have had: that proximity to thebattery matters. Indeed, a pretest question posed muchearlier in the course concerned a circuit like that ofFig. 3(b), but with battery 3 replaced with a bulb. In thatsituation, about 25% of students reckoned that this bulbwill get more current than bulb B because of its closerproximity to battery 2.

    Finally, 10% ranked the current through battery 2 greaterthan that through battery 3, and the current through battery4 as greater than that through 5. One student wrote, this isbecause the current through these batteries [i.e., 3 and 5]experiences no resistance. This student focuses on oneaspect that was important when adding bulbs in parallel,but in doing so apparently treats the original battery asdifferent from the battery that was added.

    In summary, we have found that students have a numberof misconceptions when dealing with circuits containingmultiple batteries in a single loop. Specifically, a largenumber of students believe that bulbs in circuits withbatteries in opposite orientation will not light. They eitherstate that the battery in opposite orientation does not permitcurrent in these circuits or that the circuits are incomplete.It is also evident that old notions, such as the batterysupplying a (constant) current, resurface in this newcontext.

    Additionally, we find that students tend to basetheir answers primarily on current and resistance when

    analyzing these types of circuits. This cannot be termed amisconception, but it suggests a lack of understanding andconfidence in conceptual reasoning based on voltage.

    C. Curriculum

    To appreciate the pretest questions asked on circuits withmultiple batteries in multiple loops, we describe the cur-riculum developed to address the difficulties identified inthe pretests on multiple batteries in single loops here.Students start by investigating the addition of batteries inseries (in the same orientation) in four different elementarycircuits: a single-bulb circuit, a two-bulb series circuit, atwo-bulb parallel circuit, and one with three bulbs, two ofwhich are in parallel. The students devise a rule like:WhenI add a second battery in series with another battery (in thesame orientation), the brightness of all bulbs in the circuitincreases. Students then investigate how adding batteriesin parallel affects bulb brightness, allowing them to formanother rule: When I add a second battery in parallel withanother battery (in the same orientation), the brightness ofall bulbs in the circuit remains the same. Students thencarry out three more experiments in which they add bat-teries in opposite orientation in single-loop circuits. Theydiscover that when a battery is added in opposite orienta-tion, it effectively cancels a battery already connected inthe circuit, and that the order in which batteries and bulbsare connected does not affect bulb brightness or the currentin the circuit. A small number of structured experimentsand exercises suffices here for almost all students; they findit easy to incorporate the new rules into their model.The focus of this part of the curriculum is on current and

    bulb brightness, not on voltage. If this part of the curricu-lum were used without the multiple-loops curriculum de-scribed in Sec. IV, we would add a number of exercises thatexplicitly make students think of the effect of addingbatteries in terms of voltage. We would not envisagemany problems for students if this approach were adopted.

    D.Multiple batteries in a single loopPost-test analysis

    We post-tested the effect of adding batteries in series indifferent orientations as part of a long sequence of questionsthat probed other aspects of the curriculum at the same time,either by adding an extra battery or reversing its orientation.Over 90% of answers were correct in each case.A post-test question on the addition of batteries in

    parallel with other batteries, shown in Fig. 4, was givenduring three years, with slight variations in the orderingand labeling of batteries. The questions are directly com-parable, which gives us a pool of 50 answers to work with.Here batteries are connected both in parallel with eachother and in series, a configuration not encountered in thecurriculum.In the first question, we asked students to rank all circuit

    elements by current when both switches are open. AsTable III shows, just over half the students answered the

    SMITH AND VAN KAMPEN PHYS. REV. ST PHYS. EDUC. RES. 7, 020115 (2011)

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  • question completely and correctly, while another 10%ranked the batteries and bulbs correctly but separately. Afurther 10% did not rank the batteries at all; the remaining30% gave different answers that are difficult to categorize.

    In the second question, students were asked to state whathappens to the brightness of bulb D when switch 1 isclosed. More than 60% correctly stated that its brightnesswould not change, and about one-third stated that it wouldincrease. This is a significant improvement on the corre-sponding pretest data (20% stated that bulbs A and Bwould be equally bright; see Table II). However, only20% ranked the currents through all circuit elements cor-rectly in this case. Another 20% thought that battery 2would get no current; this answer is consistent with the factthat the circuit appears to be unaffected, but does notincorporate the symmetry arguments discussed in the cur-riculum. Fifteen percent made an incorrect generalizationand thought that all three batteries would receive equalcurrents. Surprisingly, 10% stated that the current wouldsplit in two equal proportions between batteries 1 and 2 aswell as between bulbs B and C and bulbs D and E, butnevertheless ranked the currents through the parallel

    batteries greater than the currents through the parallelbulbs. This may be an artifact of how the question wasposed, but we really cannot explain this to our own sat-isfaction, as none of these students explicitly stated whythe bulb and battery currents would be different.

    IV. MULTIPLE BATTERIES IN MULTIPLE LOOPS

    A. Batteries in parallel with othernetworksPretest analysis

    To investigate how students deal with multiple batteriesin multiple loops (i.e., batteries that are connected witheach other neither in series nor in parallel), we asked themto rank the bulbs in the circuits of Fig. 5 by brightness. Toanswer the question completely, students must reason bothin terms of current and voltage.The most prevalent answers are listed in Table IV. All

    students appeared to consider the three-battery circuit interms of the difference with the two-battery circuit (i.e., theaddition of an extra battery); none seemed to look at thecircuit as consisting of two parallel branches (one consist-ing of bulb D and one consisting of batteries 3 and 4 andbulb C) connected to battery 5.Even though 35% of students ranked the bulbs correctly,

    none provided correct reasoning. Eleven of these 13 stu-dents reasoned that the addition of a battery in parallel doesnot affect the brightness of bulbs in a circuit. Most of thesestudents based their reasoning on their observations thatconnecting a battery in parallel to a single-battery, single-bulb circuit did not affect bulb brightness, and have in-correctly generalized this rule.Eight students stated that bulb C would be brighter than

    bulbs A and B (which have the same brightness), and thatthese are brighter than bulb D. They treated battery 5 as acircuit element with characteristics varying from a wire toa bulb, with otherwise correct reasoning, for example:All the current through the two batteries goes through C.

    A and B are arranged in series so voltage across them ishalved between them. Their brightness is less than C. D isin parallel with battery 5. This branch has little resistanceand the current will go through battery 5 and D will beshorted out.As there is less resistance due to [battery 5] connected in

    parallel across bulb D it has less voltage across it than bulbC and is therefore less bright than bulb C.

    A

    B

    1

    2

    3

    4

    5

    C

    D

    FIG. 5. Circuit diagrams used in the multiple batteries inmultiple loops pretest. Students are asked to rank the bulbs bybrightness.

    TABLE III. Bulb rankings in multiple batteries in single loopspost-test of Fig. 4.

    Ranking Total (N 50)When switch 1 is open:

    1 3 A G> B C D E> 2 F 0 50% (26)Batteries and bulbs ranked separately but correctly 10% (6)

    Batteries not ranked, bulbs ranked correctly 10% (4)

    Other 30% (14)

    When switch 1 is closed:

    Brightness of bulb D is unchanged 60% (31)

    Bulb D gets brighter 35% (17)

    Bulb D gets dimmer 1 2 B C D E> F 0 20% (9)1 3 A G> B C D E> 2 F 0 20% (9)3 A G> 1 2> B C D E> F 0 10% (6)The current through all batteries is equal 15% (7)

    Other 40% (19)

    A

    B C

    ED

    1 2

    3

    F

    G

    S1

    S2

    FIG. 4. Circuit diagram used in the multiple batteries in asingle loop post-test.

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  • While it is in itself encouraging that students makecomparisons to other circuit elements, and then reasonconsistently, we also note that student reasoning is stillpredominantly based on current and resistance. However,voltage does begin to make more of an appearance, eventhough it does not appear to have been fully incorporatedinto their model.

    Many others appeared to treat the battery as a source ofcurrent. For example, four students reasoned that bulb Dwould be brightest, since it received current from threebatteries:

    D>C A B. Bulb D receives the current throughthe 2 batteries in series and also the one in parallel.

    Six students thought that bulbs A and B would beequally bright, and bulbs C and D would be equally brightbut dimmer than bulbs A and B, by reasoning that battery 5opposes batteries 3 and 4. This may be the result ofincorrectly generalizing from their experience with mul-tiple batteries in single loops.

    B. CurriculumElectric potential

    To help address the issues identified in the pretest, weintroduced the concept of potential in the by-now familiarcontext of single-loop circuits. In a first version of thecurriculum, we only introduced potential when it was es-sential (see Sec. IVC). However, introducing it in thesingle-loop curriculum makes it easier for students to linkpotential to circuits containing multiple batteries in singleloops, which they understand well in terms of current andresistance. Thus, it both completes the prior curriculum andbridges it with the more complex circuits that are to follow.

    Students use the circuits of Fig. 6 to find that all voltagesacross individual circuit elements can be deduced fromknowing the potential differences between one of thepoints (say, A) and all others. We then introduce the con-cept of electric potential (at any point P) by defining it asthe potential difference between points P and A. Thestudents arrive at the following key conclusions:

    the potential at the positive terminal of a battery isgreater than the potential at the negative terminal ofthe same battery by an amount equal to the batteryvoltage;

    the potential across a bulb changes by an amountequal to the voltage across the bulb;

    the current through a bulb is always in the directionfrom high potential to low potential;

    the current through a battery can be in eitherdirection.

    C. CurriculumMultiple batteries in multiple loops

    The curriculum described in Sec. IVB contains all therules the students need to predict and explain the behaviorof many multiple-loop circuits qualitatively. To give anexample, it is a significant challenge to enable students tounderstand that, and analyze how, the two circuits shown inFig. 7 behave differently. The concept of potential helpsthem achieve this.In the Physics by Inquiry curriculum [9], students have

    seen that, if the switch is closed in the circuit of Fig. 7(a),the parallel bulb A is not affected (in that case, the boxrepresents any arrangement of bulbs, excluding those thatwould cause short-circuiting). In the circuit of Fig. 7(b),bulbs B and C do change in brightness when the switch isclosed.Students find that these rules still hold when the boxes

    are allowed to contain batteries, with some caveats. Forexample, in the circuit of Fig. 7(a), the batteries inside thebox must not be arranged in such a way that bulb A iseffectively short-circuited (for example, if the box containsjust one battery). Likewise, the brightness of bulb B couldadventitiously remain unchanged if the box contains asingle battery.Regardless of whether the curriculum is used with or

    without prior exposure to the Physics by Inquiry curricu-lum [9], the distinction between the two kinds of circuits

    A

    C

    D

    EB

    A

    C

    D

    EB

    A

    C

    D

    EB

    FIG. 6. Circuit diagrams used to develop the idea of potential.Students note the voltages across different elements and theorientation of the voltmeter leads.

    (a) (b)

    A

    B

    C

    FIG. 7. Parallel connections (a) directly across the battery,(b) in series with another circuit element. The boxes representany arrangement of bulbs and batteries.

    TABLE IV. Bulb rankings in the multiple batteries in multipleloops pretest of Fig. 5.

    Ranking Total (N 40)A B C D 35% (13)C> A B> D 20% (8)A B> C D 15% (6)D> A B C 10% (4)A B C> D 10% (3)Other 15% (6)

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  • helps to convey that a battery maintains a constant voltagebetween its terminals. Of course this observation is valu-able in its own right, but it also appears to help studentsaccept at a visceral level that the current through identicalbatteries depends on how they are connected.

    To get students thinking about current through andvoltage across a battery, they revisit the two circuits givenin Fig. 8. From the unchanged bulb brightness, they inferthat on closing the switch the voltage across the battery andbulb remains constant. In the circuit of Fig. 8(a) the currentthrough the battery must double, but in that of Fig. 8(b) thecurrent through the original battery halves (by symmetry)[15]. Students also consider that if they just looked at theouter loop of the latter circuit, they might conclude thatthere is no current in the circuit at all. Thus, early onstudents are encouraged to take a view of the circuit as asystem, and not to apply reasoning that is sound for oneloop incorrectly to a different loop.

    The main body of the curriculum revolves around thecircuit of Fig. 9(a). The students already have all the toolsnecessary to predict what will happen when the switch

    closes, but many are surprised at their observations whenthey verify their predictions. Students typically have notapplied their model for potential described in Sec. IVBspontaneously; but when explicitly asked to do so, theyreadily find that VR 2V0 VP V0 VQ, whereV0 is the battery voltage. From this they deduce that bulb Bis brighter than bulb A, and that the direction of current isfrom Q to R, and from Q to P. By applying Kirchhoffsjunction rule, students infer that all current flows throughbattery 1, splits at Q, and recombines at P. Note thatwithout the rules for current through a bulb described inSec. IVB, students could come up with four directions forthe current to explain the observed relative brightness ofbulbs A and B. Two of these are given in Fig. 9(b) (correct)and Fig. 9(c) (incorrect); the other two possibilities have allthree currents reversed.Students next encounter the circuit of Fig. 9(d). They now

    find that VR VQ VP V0, and hence that thereis no current through bulb B and battery 2. The difference inbehavior of the two circuits of Figs. 9(a)9(d) is verystriking and memorable. To embed these new ideas morefirmly, students then investigate three more circuits (notshown in this paper) where a third battery has been addedin series with battery 2.

    D. Post-test analysis

    We present data on two post-test questions examiningmultiple batteries in multiple loops. One concerned acircuit quite similar to the circuits presented in the curric-ulum, the other was quite different so that we could probethe depth of students understanding further.The first post-test question under discussion asked stu-

    dents (N 31) to rank the relative brightness of the bulbsin the circuit shown in Fig. 10. Table V reveals that almostall students (95%) obtained the correct answer; just overhalf of those used similar reasoning to that outlined inSec. IVC, while a further 25% very likely did so but didnot make all steps explicit (for example, they gave consis-tent values for potential at various points and stated that thevoltage across bulbs C and D would be greater than thevoltage across bulbs A and B, without explaining why).Twenty percent used reasoning based on application ofKirchhoffs loop rule to just one or two loops. These

    (a)

    A

    B

    1 2

    (d)

    A

    B

    1 2

    (c)

    A

    B

    1 2

    (b)

    A

    B

    1 2

    i1

    iB

    iA

    i1

    iB

    iA

    Q

    P

    R

    Q

    P

    R

    FIG. 9 (color online). Two circuits with multiple batteries inmultiple loops used to develop the concept of potential anddetermine the direction of current. Circuits (a)(c) are identical;in circuits (b) and (c), the length of the arrow indicates themagnitude of the current.

    3

    4

    C D

    1

    2

    A

    B

    FIG. 10. Circuit diagram used in a multiple batteries in mul-tiple loops post-test.

    (a) (b)

    FIG. 8. Circuits used to highlight the similarities and differ-ences between connecting in parallel with a bulb and battery(a) a second bulb and (b) a second battery.

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  • students concluded that, based on the left loop, the voltageacross bulbs A and B was half the voltage across a battery,while based on the right loop, the voltage across bulbsC and D was twice the voltage across a battery. Thus theyobtained an answer that is qualitatively correct but quanti-tatively in error.

    In the second part of the question, students were asked toindicate the direction of current through the batteries. Tocorrectly reason the direction of current through batteries24, students should refer to the direction of currentthrough bulbs C and D. On this basis, the current throughbatteries 24 is upwards. To determine the direction ofcurrent through battery 1, students must reason on the basisof potential while satisfying Kirchhoffs junction rule. Thisallows them to determine that the direction of currentthrough bulbs C and D is from right to left and the directionof current through bulbs A and B is downwards. Since thevoltage across bulbs A and B is half the voltage acrossbulbs C and D, they are dimmer, less current flows throughthem, and hence by Kirchhoffs junction rule the currentthrough battery 1 must flow downward.

    The direction of the current through batteries 24 wasdetermined correctly by 90% of the students, but only 25%used the reasoning based on the direction of currentthrough bulbs C and D. The majority of the remaininganswers merely stated the convention that the currentthrough the battery is from the negative terminal to thepositive terminal inside a battery. However, this appears togive too pessimistic a picture: if this were the sole basis fortheir reasoning, we would expect all 20 of these students tostate that the current through battery 1 would also beupwards, but only half of them (10) did. In all, 20 studentscorrectly stated that the current through battery 1 would bein the downwards direction, with eight providing completereasoning and another eight providing incomplete reason-ing. Four of the correct answers were based on partiallyincorrect reasoning.

    The second post-test question we consider concerns thecircuit shown in Fig. 11. Note that this question was givento only 14 students, so the statistics are only indicative.However, we include it here to show to what extent transferto different types of circuits takes place. The circuit is quite

    different from what the students had encountered in thecurriculum in that there are two branches with a bulb and abattery; not only does it look different, to obtain an answerKirchhoffs junction rule must be applied before the ruleson potential.In the first part of the question, students were asked to

    rank the relative brightness of the bulbs when the switchwas in the center position. Some 65% of students correctlystated that none of the bulbs would light when the switchwas in the center position as shown above; all used correctand complete reasoning. The remaining students noted thatbulbs A and C were connected in series but failed to takeinto account that the batteries effectively cancel each other;they stated that bulbs A and C would be equally bright (butnot off).In the second question of the post-test, students were

    once again asked to rank the brightness of bulbs when theswitch in the circuit of Fig. 11 was turned to the right,adding a battery to the circuit. While 70% of the studentsgave a correct ranking, only half of these gave completeand correct reasoning; 30% gave correct but incompletereasoning, while 20% were unclear or incorrect.In the last question, students were asked to rank the

    brightness of the bulbs when the switch in the circuit ofFig. 11 was moved to the left. Reasoning here is a littlemore intricate; algorithmically considering potentials firstand then applying the junction rule will not work. Studentsshould consider that the potential below bulb B is less thanthe potential above bulb B, so current is in the downwarddirection. By symmetry, the currents through bulbs A andC are equal and in the same direction; by Kirchhoffsjunction rule, these currents must be upward. Thus bulbB must be brighter than bulbs A and C, which are equallybright.No student used this line of reasoning, although three

    students did provide the correct ranking with incompletereasoning. All three used reasoning based on voltage; theyall stated that the voltage across bulb B was greatest, butdid not give a complete answer, for example:Bulb B will be the brightest. Parallel branches have

    equal voltages across them and the branches containingbulbs A and C have batteries on them. if the sum of the

    CA

    B

    FIG. 11. Circuit diagram used in a multiple batteries in mul-tiple loops post-test.

    TABLE V. Bulb rankings in the multiple batteries in multipleloops post-test of Fig. 10.

    Ranking Total (N 31)C D>A B 95% (29)

    Complete correct reasoning 55% (17)Correct but incomplete reasoning 25% (7)

    Correct but incorrect reasoning 15% (5)

    A B C D 5% (2)Upward current through batteries 24 90% (28)Downward current through battery 1 65% (20)Upward current through battery 1 30% (10)

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  • voltages of each element on a branch is equal to the sumsof the V of all of the elements on the next branch then B willbe brightest as it is on a branch on its own. The other 2branches have batteries on them which have a voltage i.e.,V1 V2 VA VB and V3 V4 VC VB. [. . .] thevoltages of A C are equal + they will be the samebrightness.

    V. CONCLUSION

    This research has extended the body of physics educa-tion research on students understanding of circuits con-sisting of multiple batteries in multiple loops. Pretestanalysis identified that a large majority of students areunable to explain the effects of adding batteries in singleand multiple loops. In answering these questions, studentsrelied mostly on their understanding of current and resist-ance, while reasoning based on voltage is a necessity. Eventhough the belief that the battery supplies current, albeit avariable current, to empty wires and bulbs resurfaced,some students did display an ability to model a circuitbased on their conceptual understanding of voltage, cur-rent, and resistance.

    The introduction of the concept of potential allowedstudents to systematically discover rules for how circuitswith multiple batteries in multiple loops can be modeled.Post-test results show that the majority of students wereable to apply their newly developed model to make accu-rate predictions for complex circuits. The analysis of theiranswers also revealed an increased understanding of theroles current, voltage, and resistance play in their model forelectric circuits. However, students still found it difficult totransfer their understanding to a new context.

    ACKNOWLEDGMENTS

    We gratefully acknowledge fruitful discussions withThomas Wemyss, John Thompson, Peter Shaffer, andMacKenzie Stetzer. We are also grateful to Roger Feeleyand Benjamin Pratt for using the curriculum in their in-stitutions and for their very useful feedback, and toEnda McGlynn and Mila Kryjevskaia for a thoroughreading of the manuscript. This work was supported by theIrish Research Council for Science Engineering andTechnology, Dublin City Universitys Learning Inno-vation Unit and the School of Physical Sciences.

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    resistance, the current can split among two parallel

    batteries in any proportion. For real identical batterieswith low but nonzero resistance, the current would splitin half. We have found that typically the resistance of twobatteries that appear identical is sufficiently different thatdifferences of as much as 20% may occur.

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    http://dx.doi.org/10.1119/1.1934552